Gaseous detectors for detecting ionizing radiation are well-known. FIG. 1 shows a schematic diagram of such a gaseous detector having a planar geometry. X-rays 122 entering the detector pass through an X-ray transparent cathode or window 112 and enter a drift region 118 located between cathode 112 and a mesh layer 124. The drift region 118 is filled with a material that is typically a working gas, such as a quenched noble gas mixture that absorbs X-rays. When an X-ray is absorbed in the working gas, fast photoelectrons are produced along the X-ray trajectory. Secondary electrons 125 issued from the thermalization of the photoelectrons accelerate in the drift region 118 in response to an electric potential provided by voltage source 116. The electrical potential in region 118 is selected so that it is not high enough to induce electron avalanche multiplication. After passing through the mesh 124, the electrons enter a high field amplification region between mesh 124 and resistive anode 128. The electric potential in this region is provided by voltage source 117, which produces an electric potential than is higher than that produced by voltage source 116. The electric potential in amplification region 119 is selected so that the field strength in that region is sufficient to induce electron avalanche multiplication within the working gas.
The electron avalanche phenomenon within the gas results in the formation of an electron cloud 126 that that is absorbed by the anode 128. Anode 128 is typically a layer that has no defined conductive paths, but which is a reasonably homogeneous material of predetermined resistivity. As shown in FIG. 1, the anode is connected to ground at the edges, so the electrical energy absorbed from the electron cloud eventually dissipates. However, the anode material is resistive enough that there is a temporary accumulation of electric charge in the local region of the anode 128 upon which the electron cloud is incident.
Positioned adjacent to the anode 128 to the side opposite to the incoming electron cloud is a readout structure 130 which is conventionally comprised of two orthogonal serpentine delay lines strips or pixels. As the electron cloud encounters the resistive anode 128, the deposited charge creates a capacitive coupling between the anode and the delay lines of the readout structure 130. This capacitive coupling induces currents in certain paths of the delay lines of the readout structure 130. These currents are detected by detection circuit 115, and have a temporal signature indicative of the parallel paths in which they were induced. This temporal signature may be used to determine the position of the electron cloud in the detection plane.
Gaseous detectors of this type have a number of very attractive features for imaging ionizing radiation including a large active area, low noise and high count rate capability. However, they typically require the radiation to pass through at least a centimeter thickness of gas in drift region 118 in order to achieve good detection efficiency. The thickness of region 118, in turn, introduces a non-desirable parallax error in the output.
The parallax error of a planar geometry gaseous detector, such as that shown in FIG. 1, is fundamentally limited by the detector geometry and the electric field in the drift region 118. In particular, the secondary electrons 125 will drift along the electric field lines emerging from the cathode 112. Parallax broadening occurs if the field lines do not coincide with the original X-ray photon direction. This is illustrated in FIG. 1 where an X-ray 140 strikes the detector at an oblique angle. As mentioned above, when the X-ray is absorbed in the working gas, fast photoelectrons are produced along the X-ray trajectory. However, because the X-ray travels at an angle with respect to the electric field lines, these photoelectrons are produced at different positions along the length of the detector. Secondary electrons 145 and 155 issued from the thermalization of these photoelectrons accelerate in the drift region 118 and produce avalanches 150 and 160 in the amplification region 119. The result is an asymmetric broadening of the diffraction spots. This undesirable effect becomes more pronounced at higher angles of incidence. To eliminate the parallax, the electric field lines along which secondary electrons move must emanate from a focal point which coincides with the position of the sample under study.
Some prior art attempts to overcome this problem have used a spherical conversion volume generated by a proportional wire chamber equipped with a resistive divider adapted to generate appropriate spherical equipotential surfaces within the drift space of the wire chamber.
Other conventional approaches use a radially symmetric change of the potential (spherical field) in the entrance window to the detector or in the cathode. For example, such a spherical field can be created by using a curved entrance window at constant potential. The problem with this approach is that the thickness of the conversion region changes considerably in the z direction, which is acceptable only for certain applications. In addition, a parallax error is still observed in these structures.
Replacing the parallel drift field with a radial drift field has been used in other prior art approaches. In these approaches, the parallax error is reduced, but only on a limited sensitive area of the detector, mainly near the central region.
In still another approach, the detector structure is based on a spherical gas electron multiplier, which serves as a transfer electrode with limited amplification to compensate for transparency losses.